A fossil is the remains, impression, indication, or trace of a once-living plant, animal, or other organism found naturally in rock. Fossils can be very small or very large. Fossils include bones, shells, exoskeletons of animals, and even the imprints of microbes.
Fossils can be preserved in many ways. The simplest fossil is the intact preservation of the hard parts of a plant or animal. The bones or teeth of an animal are preserved or the wood of a plant is preserved. Some dinosaur remains are preserved intact as fossils.
Another type of fossilization occurs when a buried plant or animal decomposes and leaves a residual film of carbon behind–a kind of outline of the plant or animal left in rock.
A third type of fossilization occurs when a buried plant or animal is gradually replaced by silica, calcite, dolomite, or pyrite from solutions that permeate the rock in a process called petrifaction. An example is when wood is replaced by agate or opal as the result of the action of hot, silica-bearing water. The fossilization of some wood is so complete that even the details of the cellular structure are preserved.
Paleontology is the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are more than 10,000 years old. The oldest fossils are more than 4 billion years old. The age of a fossil is determined by what is known as radiometric dating techniques.
Fossils reveal that many kinds of plants and animals, common in the past, no longer exist. Fossils also show that many living things today strongly resemble forms of life that now extinct.
Fossils can reveal past climates, that animals requiring a warm climate once lived where the climate is now very cold and vice versa.
Only a small fraction of the total number of once living things has been preserved as fossils.